高井 健一

Effects of Si and Ca addition on delayed fracture of medium carbon steels with 1400N/mm^2 strength were investigated. Silicon and calcium were added at concentrations of 0∿2.0% and 30∿70ppm, respectively. The delayed fracture characteristics were evaluated by FIP (Federation Internationale de la Precontrainte) test which is a constant tensile load test in 20% NH_4SCN solution at 323 K. In order to make clear the effect of adding Ca and Si, the fracture surfaces were examined, and the hydrogen evolution behavior, the diffusion coefficient of hydrogen, and the hydrogen content were measured. It was found that 0.5%Si steels have no effect on the time to fracture regardless of Ca content, while 1.5%Si-30ppm Ca steel has the longest time to fracture. Fractography showed that adding Ca to 0.5%Si steels did not change the intergranular fracture area fraction. However, adding Ca to 1.5%Si steels changed the fracture from intergranular fracture to microvoid coalescence fracture. As for hydrogen behavior after three months from FIP test, 0.5%Si steel released hydrogen at the peak of 500 K, while for the 1.5%Si-30ppm Ca steel the peak was at 700 K. It was suggested that hydrogen released at around 500 K was crucial for delayed fracture characteristics.